Age-hardenable aluminium alloys

ABSTRACT

This invention concerns AA5000 series alloys with the addition of Cu that can be retained in a solution treated condition after hot working, for example by hot rolling on a hot mill or by hot extruding. There is described a method of producing an age-hardenable aluminium alloy comprising the steps of: a) casting an alloy of a composition comprising the following expressed in weight percent: Magnesium : 1.0 to 4.0, Cooper : 0.1 to 0.6, Manganese : up to 0.8, Iron : up to 0.5, Silicon : up to 0.3, Chromium : up to 0.15, Titanium : up to 0.15, Balance : Aluminium with incidental impurities b) optionally homogenising the cast alloy, c) hot working the casting at an initial temperature of at least 400 DEG C to form an intermediate product, wherein at least part of the hot working is carried out whilst the casting is at a temperature above the solvus temperature of the alloy, d) cooling the intermediate product either during hot working or in a subsequent step at a rate such that at least a partially recovered or recrystallised structure is formed ant that sufficient copper is retained in solid solution in the alloy to cause an age hardening effect on the alloy if phase precipitation takes place during the alloy&#39;s subsequent thermal history, and e) optionally allowing or arranging for phase precipitation to occur in the alloy. The described method is particularly suited to the production of can end stock and sheet for automotive applications.

[0001] This invention concerns AA5000 series alloys with the addition ofCu that can be retained in a solution treated condition after hotworking, for example by hot rolling on a hot mill or by hot extruding.

[0002] In the art AA5000 series alloys are usually regarded as non-heattreatable alloys i.e. they are not regarded as age hardenable. Theaddition of Cu to these alloys renders them age hardenable, as describedin EP-A-0773303, EP-0616044 and EP-A-0645655. However these knownmethods also require a formal solution treatment.

[0003] The novel feature of this invention is the discovery that forcertain Cu—containing AA5000 series alloys sufficient solution treatmentoccurs during hot working, for example hot rolling, to render the alloysage hardenable without a further expensive solution treating step. Thisgives a very significant economic advantage especially for commodityproducts such as can end stock, automotive sheet products, or extrudedproducts such as structural sections.

[0004] EP-A-0605947 describes manufacturing can body sheet using twosequences of continuous operations. The described additional steps ofuncoiling the hot coiled sheet, quenching the sheet without intermediatecooling, cold rolling and re-coiling the sheet are required, but theseadditional steps are not needed in the method of the present invention.

[0005] WO-A-99/39019 describes a method for making can end and tab stockbut annealing of the sheet is required as a separate operation after hotrolling which is not needed in the method of the present invention.

[0006] WO-A-98/01593 describes a process for producing aluminium alloycan body stock but again a separate annealing step is required.

[0007] JP-A-100121179 describes aluminium alloy sheet for carbonatedbeverage can lids but a formal solution heat treatment is required,which is not needed in the method of the present invention.

[0008] U.S. Pat. No. 5,655,593 describes aluminium alloy sheetmanufacture in which the hot strip is cooled rapidly to minimise theprecipitation of the alloying elements. This teaching of rapid coolingis contrary to that of the present invention.

[0009] U.S. Pat. No. 3,464,866 describes a process for obtainingaluminium alloy conductors but again teaches rapid cooling.

[0010] In accordance with the present invention there is provided amethod of producing an age-hardenable aluminium alloy comprising thesteps of:

[0011] (a) casting an alloy of a composition comprising the followingexpressed in weight percent: Magnesium: 1.0 to 4.0 Copper: 0.1 to 0.6Manganese: up to 0.8 Iron: up to 0.5 Silicon: up to 0.3 Chromium: up to0.15% Titanium: up to 0.15%, preferably up to 0.05% Boron: from 0 up to0.05, preferably up to 0.01 Balance: Aluminium with incidentalimpurities

[0012] (b) optionally homogenising the cast alloy,

[0013] (c) hot working the casting at an initial temperature of at least400° C., to form an intermediate product, wherein at least part of thehot working is carried out whilst the casting is at a temperature abovethe solvus temperature of the alloy,

[0014] (d) cooling the intermediate product during hot working or in asubsequent step at a rate of less than 5° C./min such that at least apartially recovered or recrystallised structure is formed and thatsufficient copper is retained in solid solution in the alloy to cause anage hardening effect on the alloy if phase precipitation takes placeduring the alloy's subsequent thermal history, and

[0015] (e) optionally allowing or arranging for phase precipitation tooccur in the alloy.

[0016] Preferably after the said hot working step the intermediateproduct is generally maintained at a temperature below the solvustemperature of the alloy, provided that if the intermediate product isheated above the alloy's solvus temperature then cooling thereof iseffected at a rate less than 2° C./sec.

[0017] By the term “the solvus temperature of the alloy” is meant thetemperature below which under equilibrium conditions the copper beginsto be removed from solid solution to form a precipitate. However, as tothe rate of copper removed that will depend on the kinetics of thereaction.

[0018] The precipitation phase if formed is believed to be S phase (anAlCuMg phase) or its metastable precursors.

[0019] The alloy may be cast by DC casting to form an ingot or bycontinuous casting, for example in a belt caster or a twin roll castingmachine, to form a sheet.

[0020] The cast and preferably homogenised alloy can be extruded but forthe production of can end stock it is generally hot rolled. Aftercasting the preferred steps are:

[0021] optionally homogenising the casting at a temperature of at least480° C., and preferably 500 to 600° C., so that substantially all of themagnesium and copper in the casting are in solid solution,

[0022] optionally hot rolling the casting, optionally with re-heating ofthe casting to above the alloy's solvus temperature, preferably at least450° C., to take substantially all of the magnesium and copper presentinto solid solution,

[0023] hot rolling the casting with a rolling mill entry temperature ofthe casting of at least 400, and preferably from 450° to 580° C.,

[0024] continuing rolling the casting to the desired thickness to form asheet so that at least part of the rolling reduction is carried outabove the solvus temperature of the alloy and cooling the alloy, eitherwhile rolling or subsequently, slow enough so as to form at least apartially recovered or recrystallised structure but fast enough toensure that sufficient of the Cu is retained in solid solution toprovide an age hardening effect if a subsequent precipitation treatmentis carried out,

[0025] optionally cold rolling the hot rolled sheet, and optionally agehardening the cold rolled alloy, wherein preferably after the essentialhot rolling step the rolled ingot is always maintained at a temperaturebelow its solvus temperature.

[0026] During cold rolling, the metal temperature generally rises toabout 100-200° C. as it is passed through the mill. Conventionally aftercold rolling, the metal is coiled and being so massive the coiled metaltakes a long time to cool down to room temperature. Phase precipitationand hardening can occur during this cooling down period without the needforcibly to cool the coil. Additional cooling can, however, be used ifrequired. If desired after cold rolling re-heating can be effected ifdesired, for example to control the amount of cold work in the alloy. Ifthis re-heating takes the alloy above its solvus temperature thencooling is preferably effected at a rate less than 2° C./sec to avoiddistortion or to avoid the need for a separate quench stage.

[0027] As an alternative to batch DC casting, the alloy could be castcontinuously by for example belt casting or twin roll casting. Thesetechniques allow thin strip to be produced of a thickness of generallyas low as 5 mm, and sometimes as low as 2 mm. Such thin cast strip mayor may not require homogenisation before hot rolling since it tends tocool so quickly that the Cu and Mg present are likely to remain in solidsolution.

[0028] The casting could be extruded using direct or indirect extrusion.Preferably the casting is homogenised as described above and then cooledto room temperature before being re-heated to 400 to 500° C. forextrusion. Alternatively the casting can be cooled directly from itshomogenisation temperature to the desired extrusion temperature.

[0029] The extrudate is cooled preferably with still air or with forcedair. If desired, the extrudate can be reheated to above the solvustemperature of the alloy and then cooled at a rate of less than 2°C./sec. This re-heating treatment may be needed for texture and/or grainsize control. After extrusion the extrudate is generally stretched byabout ½to 2% and then aged.

[0030] The present invention has particular applicability for theproduction of can stock, especially can end stock (CES) which possessesa combination of high strength and formability. The combination ofcomposition and process of the present invention overcomes many of themanufacturing difficulties of the conventional AA5182 sheet currently inuse and is capable of producing CES at lower cost. It also improves thesubsequent performance of the can end, most notably its scorelinecorrosion resistance. The invention is particularly suitable fordowngauging to produce lighter weight can ends, i.e. gauges down to say0.150 mm.

[0031] For the production of can end stock, the preferred method is tocast an ingot, homogenise it, and hot roll to, say, 2 mm to form strip.A key aspect of the invention is that the strip does not need anadditional solution heat treatment step. Furthermore, even if it does,the material does not need to be rapidly cooled, e.g. does not need tobe quenched into water; the cooling is generally air cooled (possibleforced air) . The coil is then cold rolled to final gauge and lacquered.

[0032] The range (in weight percent) for the principal elements overwhich this invention is operable is: Magnesium: 1.0-4.0 wt. %,preferably 2.0-4.0, still more preferably 2.5 to 4.0% Copper: 0.1-0.6wt. %, preferably 0.2-0.5, still more preferably 0.2 to 0.4% Manganese:up to 0.8 wt. %, preferably up to 0.6, more preferably up to 0.5, stillmore preferably up to 0.4%. For some alloys a minimum Mn content of 0.1%is preferred. Iron: up to 0.5 wt. %, preferably 0.1-0.3% Silicon: up to0.3wt. %, preferably up to 0.2% Chromium: up to 0.15%, preferably traceTitanium: up to 0.15, preferably up to 0.05% Boron: up to 0.05,preferably up to 0.01% Carbon: up to 0.05, preferably up to 0.01%

[0033] For grain refining of the casting either TiB₂ or TiC can be used,but generally not together.

[0034] The present invention will now be described in more detail withreference to the accompanying drawings in which:

[0035]FIG. 1 shows a thermodynamic calculation of the solvus temperaturefor S-phase precipitation in Al-x%Mg-y%Cu-0.25Mn-0.2Fe-0.12Si,

[0036]FIG. 2 shows the conductivity changes (%IACS) during isothermalannealing of an Al-3Mg-0.4Cu-0.25Mn-0.2Fe-0.12Si alloy after solutionheat treatment and cold water quenching,

[0037]FIG. 3 shows the conductivity changes (%IACS) during isothermalannealing of an Al-3Mg-0.4Cu-0.25Mn-0.2Fe-0.12Si alloy after solutionheat treatment, cold water quenching and cold rolling, and

[0038]FIG. 4 are curves showing the effect of time and temperature onthe extent of recrystallisation during isothermal annealing of anAl-3Mg-0.4Cu-0.25Mn-0.2Fe-0.12Si alloy after solution heat treatment,cold water quenching and cold rolling.

[0039] The theoretical basis for the present invention is as follows:

[0040] The basic premise is to select an alloy composition which willenable solute to be kept in solid solution during cooling from hotrolling temperatures (250° C. to 400° C., say). The strip is thenprocessed to bring out a precipitation hardening phase which providesextra strength. This precipitation forms preferentially on thedislocation structure introduced during cold deformation In the case ofCES this cold deformation is cold rolling, for extrusions it isstretching, and for sheet it is during forming of the sheet when it isfabricated into a component.

[0041] Although there is a thermodynamic driving force for the solute tobe removed from solid solution during hot working and subsequentcooling, the nucleation and diffusion effects are such to keep asubstantial amount of solute in solution, i.e. ‘missing the nose of thec-curve’. Accompanying FIG. 1 shows a calculation of the solvustemperatures for a range of Al—Cu—Mg alloys. This shows that the solutewill stay in solid solution above the temperatures indicated. Thus, thesolute can not start to come out of solid solution until the strip is ator below this temperature. It should be noted, that even if the solutedoes start to come out of solid solution, there may still be sufficientsolute available to provide an appreciable strengthening effect duringsubsequent processing.

[0042] The conductivity has been determined for a3Mg-0.4Cu-0.25Mn-0.2Fe-0.12Si alloy (wt. %) to demonstrate that for thistype of alloy there is a barrier to nucleation and growth of theprecipitates which can be commercially exploited to provide an improvedbalance of strength and formability. Accompanying FIG. 2 shows theeffect of isothermal ageing on the conductivity of a full solution heattreated and cold water quenched material subject to isothermal ageing.This shows that at temperatures below the solvus the conductivityincreases (indicating Cu along with Mg removed from solid solution), butthat at lower temperatures the precipitation becomes difficult. Thus,the solute can be kept in solid solution if the strip can be cooled tothese temperatures sufficiently rapidly.

[0043] If there are dislocations present then the conductivity rise ismore rapid, since the precipitating phase is believed to be S-phase (anAlCuMg phase), or its metastable precursors which is well-known tonucleate preferentially on dislocations. To demonstrate this, a furtherset of isothermal ageing experiments have been performed on the samealloy, but after solution heat treatment, cold water quenching and coldrolling. This is shown in accompanying FIG. 3. In this case theconductivity drop starts to occur after a few seconds. This shows theimportance of passing through this temperature regime without largenumbers of dislocation present, since if the phase nucleates at thesehigh temperatures it is likely to be relatively coarse and providelittle strengthening. The example shown is an extreme example since thestrip was cold rolled to introduce a high dislocation density prior toageing. In hot deformation the dislocation density is lower for a fixedlevel of macroscopic strain, thus providing fewer sites for nucleationof the precipitates.

[0044] For the production of CES the hot rolling conditions are selectedto ensure that the hot rolled sheet recrystallises on or before coilingor very shortly thereafter. Preferably the sheet is fully recrystallisedresulting in a low dislocation density. Recrystallisation is encouragedby arranging for the minimum temperature of the sheet as it exits fromthe rolling mill to be 250° C., preferably 270° C. and more preferably300° C. and/or arranging for the cooling rate of the sheet to besufficiently slow to allow time for the sheet to recrystallise when inits coiled form or during coiling. In a conventional mill the coilingtemperature is approximately the same as the exit rolling milltemperature. Where additional cooling means are provided after the millthe minimum coiling temperature should be in the range of minimum millexit temperatures mentioned above. In practice acceptable cooling ratesare found to be of the order of 0.1 to 10° C./minute and preferably 0.2to 5° C./minute over the temperature range of 400-200° C. There is noneed to uncoil the sheet during cooling in order, for example, to quenchit.

[0045] An indication of the time required to achieve recrystallisationhas been determined for a 3Mg-0.4 Cu-0.25Mn-0.2Fe-0.12Si alloy (wt. %).This material was solution heat treated, cold water quenched and coldrolled 50%. Isothermal heat treatments were performed to determine theextent of recrystallisation, as shown in FIG. 4. This shows that afterthis deformation, full recrystallisation is possible within a fewminutes at temperatures in excess of around 320° C. It should be notedthat the precise details of the recrystallisation kinetics will dependon the deformation conditions and the material microstructure.

[0046] A high rolling mill exit temperature encourages precipitation ofS phase or its precursors while the strip or coil is cooling. Coolingmore quickly can counter this and prevent precipitation but if the exittemperature becomes too high, the cooling rate required is too fast tobe practically useful. To take maximum advantage of the rapid coolingduring hot rolling, the upper limit to the mill exit temperature,especially for the alloys richer in Cu and Mg, should preferably belower than the solvus temperature of the alloy. FIG. 1 gives anindication of the solvus temperature as a function of the Mg and Cucontents. Preferably the maximum exit temperature should be between 340°C. and 360°, although up to 380° C. is possible for some alloys.

[0047] It is important to note that the location of the nose of thec-curve for these alloys when recrystallised varies with the compositionof the alloy. For example, for the alloy referred to in FIG. 2, the noseof the curve is located at a time of around 100 to 1000 seconds. Fordilute alloys the nose is moved to longer times whilst for moreconcentrated alloys the nose is moved to shorter times. The timeindicated in FIG. 2 compares with times of between 1 and 100 seconds forconventional age hardening systems such as AA7075, AA2017, AA6061 andAA6063. For the alloys described in the present invention, this provideslonger times at temperatures below the solvus temperature in which tocool the strip and still maintain the Cu (and Mg) in solid solution. Forthis preferred alloy of FIG. 2 it has been found that a cooling rate of1° C./min and preferably 5° C./min is sufficient substantially to missthe nose of the c-curve and provide a substantial age hardening responseduring subsequent processing. This cooling rate can be achieved by, forexample, forced air cooling of a coil. Previous art regarding solutionheat treatment of these Al—Mg—Cu alloys teaches that, not only is aseparate solution heat treatment stage required, but that the strip mustbe quenched with a cooling rate of 2° C./second or faster. For thepresent invention it has been found that neither of these steps need tobe used, thereby providing a lower cost manufacturing route for thesealloys. Likewise no separate annealing step is needed after the hotworking step and before the cooling step.

[0048] This solute is then used to give a significant precipitationhardening effect during subsequent thermomechanical processing. Duringsubsequent cold (or warm) deformation of the strip an increaseddislocation density is introduced giving enhanced nucleation sites forthe strengthening phase. This deformation may not be needed for allapplications of this invention, since for these compositions it is knownthat the precipitation can also occur in the absence of dislocations,albeit at slower rates. The precipitating phase is believed to beS-phase which can form as needles or rods on the dislocation structure.In the case of CES this precipitation could occur during a separateageing step or during the thermal history which the material wouldexperience during deformation in, for example, strip rolling.

[0049] As shown above, it may be important to achieve rapidrecrystallisation in order to remove the dislocations from the materialas it cools. Mn can be added as a strengthening element and to controlgrain size and is therefore desirably kept as high as possible. However,Mn inhibits recrystallisation after hot rolling or during annealing, andso a maximum Mn content of 0.4% may have to be set in order to achievefull recrystallisation for some alloys under certain conditions. Formany of the alloys to assist in controlling the grain size of therecrystallised sheet, it may be desirable to have a minimum of at least0.05% Mn and preferably at least 0.1% Mn present in the alloy.Recrystallisation may also be important for crystallographic texturecontrol in CES, but this may not be necessary if the can end tooling ismodified to take significantly higher levels of earing into account.Crystallographic texture control can also be important for automotivesheet formability; another potential application of this invention.

[0050] Another feature of the composition used in the present inventionis the importance of having low Fe and Si in the alloy, since this willprevent the presence of excessive numbers of coarse constituentparticles in the sheet. These form during solidification and cannot befully dissolved during homogenisation of the ingot. Although they breakup during rolling, their presence is sufficient adversely to affectformability. Since this invention has been found to produce improvedformability over existing AA5182 CES, the strip may be able to toleratehigher levels of these elements, thus reducing cost. Tolerance of higherlevels of Si and Fe may allow greater use of recycled aluminium scrapand this is another important aspect of this invention. Up to 0.5% Femay be tolerated in the alloy and preferably up to 0.3% Fe. The minimumamount of Fe present will be dictated by cost and there is unlikely tobe less than 0.1Fe. Silicon up to 0.3% may be present, preferably up to0.2%.

[0051] Another advantage over conventional AA5182 CES is that the lowerMg content will also make the can end less susceptible to stresscorrosion cracking (SCC), which can lead to catastrophic failure of theend under the stressed conditions which are encountered in thepressurised can. The invention described here will make the end lesssensitive to these conditions, since the lower Mg content reducesbeta-phase precipitation, which has been linked to SCC. Avoidance of SCCis also important in many other applications including car body sheet.

[0052] CES is currently made from AA5182 and gets its strengthpredominantly from a combination of solute hardening and strainhardening. This makes it difficult to roll and gives a relatively highmanufacturing cost.

[0053] The alloy used in the present invention has lower strength duringthe rolling operations, but develops its strength during subsequentthermal exposure during fabrication. Thus there is the benefit ofrolling a lower strength sheet, but still enabling the desired sheetproperties to be obtained ultimately. It is also possible to produce ahigher strength sheet suitable for downgauging without a reduction inrollability (higher rolling loads, more difficulties in performing therolling operation) encountered in higher Mg containing alloys such asAA5182 and AA5019A.

[0054] The present invention is also applicable to production of lowcost automotive sheet where the material could be used in the hot rolledcondition (Direct Hot Roll to Gauge), thereby potentially avoiding theneed to solution heat treat the sheet. Alternatively, the sheet could becold rolled to gauge, as for CES, with a final continuous anneal toimpart the formability required for this application and to take thesolute into solution. Cooling after annealing should be sufficientlyrapid to retain substantially all of the solute in solution. Ageingcould be carried out in a separate operation before or after forming,for example during the paint bake stoving of the automotive part.

[0055] Some embodiments of the present invention will now be describedby way of example:

EXAMPLE 1

[0056] An alloy of the following composition was cast as a 225 mm×75 mmcross section DC ingot; Magnesium  3.0 wt. % Copper  0.4 wt. % Manganese0.25 wt. % Iron 0.20 wt. % Silicon 0.12 wt. %

[0057] Balance aluminium with incidental impurities. The ingot was notgrained refined during casting and as a consequence the Ti level was0.0018% and B was less than 0.0001%.

[0058] This was homogenised for 2 hours at 540° C. (50° C./hr heatingrate), followed by laboratory hot rolling to 6 mm. During this rollingstage the temperatures were only about 100-200° C., so the strip wasre-solution heat treated to bring about full recrystallisation and toput the solute back into solid solution. This reproduces solute levelsmore like those which would be found during rolling on an industrial hotline (but prior to coiling).

[0059] Different heat treatments were then applied at this gauge. Thestrip was either solution heat treated (SHT) (5 minutes at 550° C.) andcold water quenched (CWQ) or it was solution heat treated and then aircooled to temperatures in the range 300 to 340° C. and then cooled at 1°C./min. Conductivity was measured at this stage to determine how muchsolute remained in solid solution. These conditions were selected tosimulate the conditions which might be expected to exist duringcommercial use of this invention. Until the strip temperature dropsbelow the solvus temperature for the alloy the S phase therein cannotprecipitate and therefore the Cu (and Mg) would be substantially insolid solution. The strip could then be quenched at the end of hotrolling or, preferably, cooled after coiling. During this process thestarting temperature could be in the range 300 to 340° C. and a typicalinitial cooling rate would be 1° C./min. The temperature range betweenthe solvus temperature (about 390° C. for the alloy) and the coilingtemperature is passed through very quickly since this is when the stripmight typically be in the hot tandem mill and, hence, there is lubricantapplied to the strip which acts as a coolant. This phase was simulatedusing the air cool from the solution heat treatment temperature.

[0060] The strip was then cold rolled to 0.24 mm and given a simulationof a coil cool down to ambient temperature from 150° C. at 0.4° C./min.It was then given a simulation of a lacquer curing cycle for 3 minutesat 205° C. Tensile testing was performed at each stage of the treatmentand the results compared with results on conventional AA5182 CESmaterials processed in the laboratory.

[0061] The effect of strength development was also studied at variousstages of the laboratory simulation of the CES production route. Anexample is given below for this alloy which has been solution heattreated at 2 mm and rolled to 0.20 mm gauge. This is compared withAA5182 rolled in the laboratory using a simulation of the commercialroute for that alloy. The 0.2% yield strength is shown in Table 1 below.The as-rolled strength was found to be lower than AA5182, indicatingeasier rolling, and the strength drop during coiling and lacquer stovingsimulation was less, showing the benefits of precipitation hardening. Inaddition, in AA5182 CES the softest direction is usually at about 45° tothe rolling direction of the sheet (softer by about 10-20 MPa) and thisis believed to control the buckle pressure of the sheet. In thisinvention the levels of cold reduction needed to generate the desiredstrength level are lower and thus the weakest direction is likely to bethis longitudinal value. Hence, at its best, the combination of thecomposition and processing route of the present invention is capable ofproducing a strength level approximately 45 MPa stronger than existingAA5182. TABLE 1 Comparison of properties with conventional CES Condition5182 CES This alloy As-rolled at final gauge 430 MPa 399 MPa As-rolledand coil annealed 358 MPa 386 MPa As-lacquered 345 MPa 370 MPa

[0062] Conductivity results are shown in Table 2 below. This shows thatthe conductivity at the solution heat treatment stage is capable ofbeing increased from 33.1 to 35.0 if the solute is allowed to be removedfrom solid solution, but that if the material is cooled to ambienttemperature at 1° C./min from 300° C. there is only a fraction of theincrease in the conductivity (0.3% versus 1.9%). This implies that asignificant amount of the solute is kept in solid solution, even atthese cooling rates. TABLE 2 Conductivity after different heattreatments at 2 mm gauge Condition Conductivity (% IACS) Solution heattreated and CWQ 33.1 Solution heat treated and Fast Air Cooled 33.1Solution heat treated and cooled from 340° C. 33.9 Solution heat treatedand cooled from 320° C. 33.8 Solution heat treated and cooled from 300°C. 33.4 SHT, cold worked and aged 14 hours at 320° 35.0

[0063] The strength developed in these materials at final CES gaugeafter lacquer stoving is shown in Table 3 below. In this case the sheethas been rolled to 0.24 mm. This shows that sufficient solute remains insolid solution still to give an appreciable strength CES. Bend testinghas also been performed and indicates an improvement in the amount ofbending which can be performed prior to failure when compared withconventional AA5182 CES. TABLE 3 Strength developed after processing to0.24 mm after various thermal treatments at 2 mm ‘hotband’ gauge. 0.2%Proof Condition Stress (MPa) Solution heat treated and CWQ 350 MPaSolution heat treated and cooled from 340° C. 327 MPa Solution heattreated and cooled from 320° C. 329 MPa

EXAMPLE 2

[0064] An alloy of the following composition was DC cast for processingwithin an industrial plant: Magnesium  2.9 wt. % Copper  0.4 wt. %Manganese  0.1 wt. % Iron 0.20 wt. % Silicon 0.08 wt. % Balancealuminium with incidental impurities. The ingots were cast withadditional grain refiner.

[0065] The ingot were homogenised at 540° C. and hot rolled on a singlestand reversing mill to a thickness of 38 mm at which point thetemperature was around 480° C. The strip was then hot rolled through a3-stand hot tandem mill to a gauge of 2.5 mm. The conditions wereadjusted to give two different coiling temperatures in order to show theeffects at opposite extremes of this invention. In both cases the coilswere forced-air cooled, giving a cooling rate measured on the outer lapsof the coil of around 0. 7° C./min.

[0066] The cooler coil was processed to give a sidewall temperature of280-290° C. In this instance the microstructure of the strip was largelyunrecrystallised. As a consequence the solute was easily removed fromsolid solution on the pre-existing dislocation structure from the hotdeformation. The conductivity of this strip is shown in Table 4, showingthat the %IACS value is similar to that in which all of theprecipitation has been allowed to occur. Also in Table 4 is presentedthe conductivity obtained by using a still-air cool on strips of the 2.5mm thick metal at the end of the hot rolling (approximately 60° C. perminute), showing. that at these cooling rates a significant amount ofthe solute can be kept in solid solution.

[0067] The hotter coil was processed to give a coil sidewall temperatureof 330-340° C. Table 4 shows that in this case the forced air coolingleaves more solute in solid solution as a consequence of the fullyrecrystallised grain structure achieved with the higher coilingtemperature. The amount of solute in solid solution with the faster coolis even higher and approaches that of the conventional solution heattreated (SHT) and cold water quenched (CWQ) material. This shows that acooling rate of 0.7° C./min is able to keep some of the copper in solidsolution, but that more rapid cooling leaves more copper in solidsolution and yet is still fully recrystallised. Thus, cooling the coilwith forced-air from a temperature lower than 330° C. will achieve asimilar effect (i.e. more solute in solid solution), since the c-curvewill be substantially missed in that case too. The forced-air cooledcoil was cold rolled to 0.216 mm and the as-rolled tensile yieldstrength measured as 347 MPa.

[0068] Between these two limits of cooling temperature there will beeven more solute in solid solution at the end of hot rolling and thuseven higher strength sheet can be produced. TABLE 4 Conductivity afterdifferent thermomechanical treatments in an industrial plantConductivity Condition (% IACS) Solution Heat Treated and CWQ 35.4 SHT,CWQ + 24 hrs. at 310° C. 36.8 Forced-air cooled coil from 280-290° C.36.9 air cooled strip from 280-290° C. 36.1 Forced-air cooled coil from330-340° C. 36.4 air cooled strip from 330-340° C. 35.9

1. A method of producing an age-hardenable aluminium alloy comprisingthe steps of: a) casting an alloy of a composition comprising thefollowing expressed in weight percent: Magnesium 1.0 to 4.0 Copper 0.1to 0.6 Manganese up to 0.8 Iron up to 0.5 Silicon up to 0.3 Chromium upto 0.15 Titanium up to 0.15 Balance Aluminium with incidental impurities

b) optionally homongenising the cast alloy, c) hot working the castingat an initial temperature of at least 400° C. to form an intermediateproduct, wherein at least part of the hot working is carried out whilstthe casting is at a temperature above the solvus temperature of thealloy, d) cooling the intermediate product either during hot working orin a subsequent step at a rate of less than 5° C./min such that at leasta partially recovered or recrystallised structure is formed and thatsufficient copper is retained in solid solution in the alloy to cause anage hardening effect on the alloy if phase precipitation takes placeduring the alloy's subsequent thermal history, and e) optionallyallowing or arranging for phase precipitation to occur in the alloy. 2.A method as claimed in claim 1 wherein the alloy has the followingcomposition expressed in weight percent: Magnesium 2.0 to 4.0 Copper 0.2to 0.5 Manganese up to 0.6, preferably up to 0.5 Iron 0.1 to 0.3 Siliconup to 0.2 Chromium up to 0.15 Titanium up to 0.05 Boron or Carbon up to0.01 Balance Aluminium with incidental impurities


3. A method as claimed in claim 2 wherein the magnesium content is 2.5to 4.0%.
 4. A method as claimed in any one of claims 1 to 3 wherein theintermediate product has a substantially fully recovered orrecrystallised structure.
 5. A method as claimed in any one of thepreceding claims wherein the casting is homogenised before hot workingat a temperature of at least 480° C., preferably 500 to 600° C., so thatsubstantially all of the magnesium and copper in the casting are insolid solution.
 6. A method as claimed in any one of the precedingclaims wherein the casting is hot worked, optionally with re-heating ofthe casting to above the alloy's solvus temperature, and preferably atleast 450° C., to take substantially all of the magnesium and copperpresent into solid solution.
 7. A method as claimed in any one of thepreceding claims wherein the hot working step is carried out when thecasting has an initial temperature of from 450° C. to 580° C.
 8. Amethod as claimed in any one of the preceding claims wherein the alloyis DC cast.
 9. A method as claimed in any one of the preceding claimsincluding the step of cold rolling the hot worked casting, optionallywith coiling.
 10. A method as claimed in any one of claims 1 to 9wherein the hot working is effected by extrusion.
 11. A method asclaimed in any one of claims 1 to 9 wherein the hot working is effectedby hot rolling.
 12. A method as claimed in any one of the precedingclaims wherein the hot worked casting is cooled at a rate of less than1° C./min.
 13. A method as claimed in any one of the preceding claimswherein if after the said hot working step the temperature of theintermediate product exceeds the solvus temperature of the alloy thencooling of the intermediate product to a temperature below the alloy'ssolvus temperature is effected at a rate less than 2° C./sec.
 14. Amethod as claimed in claim 12 or claim 13 wherein the cooling iseffected by forced air cooling.
 15. A method as claimed in any one ofthe prceeding claims wherein no separate annealing step is carried outafter the hot working step (c)and before the cooling step (d).
 16. Amethod as claimed in any one of the preceding claims wherein the productof the method is can end stock.